Spinal manipulative therapy (SMT) is a common treatment for patients with low back pain (LBP) with evidence that it provides therapeutic benefit for some patients. Despite common use and evidence of benefit, mechanisms underlying the therapeutic effects of SMT are not well understood. Current theories focus on biomechanical and neurophysiologic changes produced by SMT as possible mechanisms, but these changes have not been sufficiently studied in patients with LBP, and prior research has not examined these changes in relation to the therapeutic response to SMT. Relating biomechanical and neurophysiologic changes to a patient's therapeutic response is a necessary step to separate therapeutic mechanisms of SMT from mere epiphenomena. This study assembles a team of investigators with experience in the measurement of biomechanical and neurophysiologic responses to SMT as well as therapeutic outcomes. Training and standardization of measurement procedures will be performed prior to data collection. Two cohorts of subjects with LBP will then be recruited;one identified as likely SMT responders based on a clinical prediction rule previously developed by the Investigators;the other identified as likely non-responders. Subjects in the responder cohort will be randomized to receive SMT or a control group. Subjects in the non-responder cohort will receive SMT. A battery of tests including neurophysiologic and biomechanical characteristics, and various clinical outcomes will be assessed before and directly after each SMT treatment. Final re-assessment will occur 1 week after the final SMT treatment to examine sustained neurophysiologic and/or biomechanical changes. The primary aim of the study is to compare immediate post-SMT changes in neurophysiologic and biomechanical characteristics between subjects who achieve therapeutic success or non-success with 2 SMT treatments. We hypothesize that changes occurring in successful subjects will differ from those occurring in non-successful subjects. Secondary aims of the study are;1) compare sustained (1-week) changes in neurophysiologic and biomechanical characteristics between subjects who achieve therapeutic success or non-success with 2 SMT treatments. We hypothesize sustained changes occurring in successful subjects will differ from those occurring in non-successful subjects. 2) compare baseline (pre-treatment) characteristics of subjects identified as likely responders or non-responders to SMT based on the prediction rule. We hypothesize that differences will be observed between groups prior to the application of SMT treatment. 3) compare changes in neurophysiologic and biomechanical characteristics between subjects likely to respond to SMT based on the prediction rule who are randomized to receive or not receive SMT. We hypothesize subjects receiving SMT will demonstrate greater change. The results of this study will help to focus future research on mechanisms underlying SMT that are related to the therapeutic response to the treatment using validated measurement procedures. PUBLIC HEALTH RELEVANCE: Low back pain is a common condition resulting in high costs and disability for society and affected individuals. Manipulation is a treatment frequently used to treat low back pain, and although some individuals benefit from the treatment, why manipulation works is presently not known. We will examine neurophysiologic and biomechanical responses to manipulation along with the therapeutic response of the patient to help to determine the most relevant mechanisms underlying the therapeutic effects of manipulation.